Fine-structure genetic map of the cysB locus in Salmonella typhimurium.

A genetic map of the cysB region of the Salmonella typhimurium chromosome was constructed using bacteriophage P22-mediated transduction. Strains bearing delta (supX cysB) mutations were employed to divide this regulatory locus into 12 segments containing a total of 39 single-site mutations. Twenty-five of these single-site mutations were further ordered by reciprocal three-point crosses. The results do not support the concept of multiple cistrons at cysB and suggest that the abortive transductants previously observed in crosses between certain cysB mutants were due to intracistronic complementation. The prototrophic cys-1352 mutation, which causes the constitutive expression of the cysteine biosynthetic enzymes, was found to lie within the cysB region itself. It is bracketed by mutations, which lead to an inability to derepress for these enzymes and result in auxotrophy for cysteine.